Note: Descriptions are shown in the official language in which they were submitted.
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THE USE OF ARGON TO PREPARE LOW-CARBON,
LOW-NITROGEN STEELS IN THE
BASIC OXYGEN PROCESS
BACKGROUND
This invention relates, in general, to a proces~
for refining steel, and more specifically, to an
improvement in the basic oxygen process wherein molten
steel contained in a vessel is refined by top blowing
oxygen into the melt, i.e. from above the melt surface.
The manufacture of steel by the basic oxygen process,
also referred to as BOP or BOF process, is well known in
the art. When low carbon steel is made by this process,
it is often subJect to contamination by atmospheric
nitrogen. Such contamination tends to cause premature
age hardenin~ of the steel, which leads to strain-aging, -
poor surface properties and undesirable appearance of
.~
the final product.
The problem of nitrogen pickup during the manufacture
of low-carbon steels has been addressed by the prior
; 20 art. Glassman, in U.S. Patent No. 3,769,000, describes
a method for excluding nitrogen from the melt by placing
.
a hood loosely over the mouth of the refining vessel.
Nitrogen from ambient air is excluded by maintaining
a curtain of carbon dioxide between the hood and the
refining vessel. Pihlblad et al, in U.S. Patent No.
3,307,937 disclose a method for excluding atmospheric
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71
nitrogen from the melt by adjusting the size of the
opening through which gas flows out at the top of the
vessel, thereby maintaining positive pressure in the
vessel with respect to the ambient atmosphere, even at
low carbon levels. Both of these approaches require
modification of the BOP vessel which is expensive and
cumbersome to utilize; consequently, neither has met
with significant commercial success.
In addition to the potential for nitrogen contamination,
a second disadvantage of the conventional basic oxygen
process is the increasing quantity of oxygen that reacts
with valuable metal as the carbon content of the melt
decreases. Several U.S. patents disclose ways of
diluting the oxygen with another gas in order to
minimize the amount of oxygen that reacts with the metal.
Such patents include Fulton et al U.S. Patent No.
3,649,246 and Ramachandran's U.S. Patent Nos. 3,594,155
- and 3,666,439. These patents deal only with the problem
of increasing the degree to which the injected oxygen
reacts with carbon rather than the metal. None are
concerned with how one might utilize a diluent to
minimize nitrogen pickup from the atmosphere during
oxygen decarburization in the BOF.
OBJECTS
Accordingly, it is an object of the present
invention to prevent contamination of molten ferrous
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metal with nitrogen during decarburization by top blowing
with oxygen.
It is another object of this invention to produce
low-carbon steels having a low nitrogen content by the
basic oxygen process.
It is still another object of this invention to
minimize the amount of nitrogen-free fluid needed to
produce low-carbon steel having a low nitrogen level.
SUMMARY OF THE INVENTION
The above and other objects, which will readily be
apparent to those skilled in the art, are achieved by
the present invention, one aspect of which comprises:
in a process for the production of low-carbon steel by
blowing oxygen into a ferrous melt contained in a
vessel or zone from above the surface of said melt, the
improvement comprising the production of steel having
low nitrogen content by:
~a) in~roducing nitrogen-free fluid into the vessel
: before the nitrogen content in the melt has reached its
minimum level, while continuing the blow with oxygen,
(b) adjusting the flow rate of ~aid nitrogen-free
fluid so that the total off-gas flow rate from the
vessel is maintained at least equal to that which would
have been produced without said nitrogen-free fluid at
the time in the refining process when the nitrogen
D-10,883
797~ .
content of the melt reached its minimum level, and
(c) continuing the injection of said nitrogen-
free fluid throughout the remainder of the oxygen blow.
During practice of the basic oxygen process it is
common to interrupt the injection of oxygen into the
melt and then reblow the melt with oxygen. Reblowing
the melt is often accompanied by a significant increase
in the dissolved nitrogen content of the melt. To prevent
this nitrogen pickup when the oxygen flow has been
interrupted the vessel should be purged by injection of a
nitrogen-free fiuid immediately prior to restarting the
injection of oxygen. Thereafter the introduction of
; nitrogen-free fluid into the vessel is resumed before
the nitrogen content in the melt has reached its minimum
level, adjusted and continued as above.
The term "nitrogen-free fluid" as used herein is
intended to mean any fluid, other than oxygen, sub-
stantially free of nitrogen or nitrogen-containing
compounds. The term includes but is not limited to
argon, helium, neon, krypton, xenon, carbon dioxide,
carbon monoxide, steam, water, hydrogen, gaseous hydro-
carbons such as methane and ethane, liquid hydrocarbons
such as kerosene and n-heptane, and mixtures thereof.
The preferred nitrogen-free fluid is argon.
The terms "low-carbon steel" and "low-nitrogen
steel" as used herein are intended to include respectively
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D-10,883
i~37971
steels having a carbon content no higher than about
0,10 percent, and steels having a nitrogen content no
higher than about 0.005 percent (50 ppm).
The term 'loff-gas" is used to mean the gases
which issue from the gas exit port or top opening of the
steel refining vessel while oxygen or oxygen and one or
more other gases are injected into the vessel in order
to refine the ferrous melt.
The term "reblow" is used to mean a subsequent
blowing of oxygen or oxygen mixed with other gas into
a BOP vessel after the initial flow of the oxygen or
oxygen-containing mixture has been stopped for any
reason. It is possible to have more than one reblow
per heat.
The preferred method of injecting the nitrogen~
free fluid is to mix it with the oxygen stream; however
alternate methods may aLso be used. The preferred
amount of nitrogen-free fluid to use when purging the
vessel prior to restar~ing the injection of oxygen is a
volume of gas, measured at 70F and 1 atmosphere pressure,
at least equal to 1/2 the vessel head space.
THE DRAWINGS
Figure 1 is a graph illustrating the final nitrogen
content N as a function of the final carbon content C
of a series of heats of metal refined by prior art BOP
D-10,883
1~7.'371
practices in a typical commercial refining system without
using the present invention. This figure illustrates
how data obtained without practicing the invention is
used to determine when nitrogen-free fluid injection
should be started.
Figure 2 is a graphic representation of the change
in off-gas flow rate F as a function of carbon content
C for same system for which data is shown in Figure 1.
This graph shows how the data, obtained without practicing
the invention, is used to determine how much nitrogen-
free fluid is to be injected.
DETAILED DESCRIPTION OF THE INVENTION
,
The band formed by curves A and B in Figure 1
shows how the nitrogen-content N of the melt varies
with percent carbon C in the melt when the present
invention is not practiced. Although all BOP systems
exhibit curves shaped similarly to Figure 1, the
numerical relationship between N and C is specific to
each BOP system and its manner of operation, and must
be plotted from data obtained during actual production
runs. The reasons for the variations from system to
system are: variations in oxygen blowing rate, lance
operating position, lance oxygen pressure, lance design,
melt weight,vessel geometry, and so on. It can be seen
that as the carbon content C decreases the nitrogen
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D-10,883
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content N also decreases until a minimum is reached, at
which point the nitrogen content begins to rise again.
The nitrogen content of the melt is used to
determine when injection of the non-nitrogen fluid should
begin in accordance with the present invention. However,
since the nitrogen content is not often regularly measured,
as is carbon content, and since nitrogen content is a
function of carbon content for a given BOP vessel, as
shown in Figure 1, the carbon content can be used to
determine the nitrogen content.
From Figure 1 it can be seen that the nitrogen
content of this particular system is at a minimum when
the carbon content of the melt is approximately 0.08
; percent.
Figure 2 shows how the off-gas flow rate F varies
with carbon content C for the given BOP refining system
at a given oxygen blowing rate without using the method
of the present invention. Approximate off-gas flow-rates
can be determined without a flow meter by preparing
; 20 a graph of carbon content versus time, determining the
rate at which carbon is removed by the slope of the
plot, and calculating the off-gas rate by assuming
that the carbon removed is converted to carbon monoxide
and that this carbon monoxide constitutes all of the
off-gas. As with Figure ls each BOP system will have
--8--
D-10,883
1~\7~
its own curve for this relationship depending upon
system characteristics and manner of operation.
While we do not wish to be tied to any particular
theory, it is a hypothesis of this invention that
nitrogen contamination in the basic oxygen process,
occuring mainly during the latter stages of decarburization
when the carbon content of the steel is low, is caused
as follows. At high carbon levels the rate of carbon
monoxide generation during the oxygen blow or decarbur-
ization period produces off-gas ratas sufficient to prevent
significant infiltration of the surrounding atmosphere
into the vessel. In addition, at high carbon levels,
the carbon monoxide boil is sufficient to sparge some
of the nitrogen that may be dissolved in the steel.
During the initial stages of decarburization therefore,
the nitrogen level in the steel decreases, as shown in
Figure 1. Beyond a certain carbon level however, as
the carbon content drops, the nitrogen content of the
melt increases. It is believed that the reason for such
increase is that as the carbon level drops, the rate of
C0 formation by the decarburiza~ion reaction and consequent
off-gas evolution drops, making it possible for atmospheric
nitrogen to enter the head space of the vessel and be
absorbed by the melt. The oxygen jet helps carry the
nitrogen down into the melt. Hence, as off-gas flow
rate decreases, as shown in Figure 2, infiltration of
D-10,883
~7g71
atmospheric nitrogen into the vessel is increased, and
eventually a point is reached in which the nitrogen
infiltrates at a rate sufficient to cause a net increase
in the nitrogen content of the steel produced.
Practice of the present invention will now be
described with reference to Figures 1 and 2. From
actual operating data one obtains N*, the minimum
nitrogen content attained during an oxygen blow for the
particular system on which the invention is to be practiced.
In Figure 1 N* is about 19 to 25 parts per million. One
then reads C*, the carbon content corresponding to N*.
From Figure 1 it can be seen that C* is 0. 08~/o . Injection
of the nitrogen-free fluid must be started no later
than when the carbon content is C*. To determine the
~ rate of injection of nitrogen-free fluid, one takes the
; carbon content at C* and reads on Figure 2 the off-gas
flow rate, F* corresponding to C*. F* is the value
below which the off-gas flow rate must not be allowed
to fall du~ing the refining process. In accordance with
this invention, the off-gas rate is maintained above
this minimum value by maintaining the rate of injection
of nitrogen-free fluid sufficient to maintain the total
off-gas flow rate above F*.
In summary, from Figure 1 one obtains the latest
point in time at which to begin injecting the nitrogen-
free fluid while from Figure 2 one obtains the minimum
.
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D-10,883
7971
amount of nitrogen-free fluid that needs to be added
in accordance with the present invention in order to
prevent contamination of the melt with atmospheric
nitrogen.
In some cases, precise instantaneous measurement
of neither the carbon content, nor the nitrogen content
of the melt is available during decarburization. It
is therefore more convenient to practice the invention
by starting injection of the nitrogen-free fluid some-
what in advance of the time when the nitrogen content
is equal to N* and the carbon content is C*. If a BOP
system has no means for constantly monitoring the off-
gas flow rate or means for controlling the off-gas rate
by varying the amount of nitrogen-free fluid that is
injected into the vessel, the invention can still be
practiced by introducing the nitrogen-free fluid at a
constant rate sufficient to maintain the total off-gas
rate at least equal to F*.
It is not uncommon during practice of the basic
oxygen process to interrupt the injection of oxygen into
the melt prior to achieving the final desired degree of
decarburiæation. When this occurs it is necessary to
reblow the melt. Similarly, it is also often necessary
to reblow the melt even though the final desired carbon
level has been reached, either because the temperature
of the molten steel is too low, or because some other
element or impurity is not at the desired level.
D-10,883
~a7~7l .
Whatever the reason, reblowing of the molten steel is
not at all uncommon. When a melt is reblown during
conventional practice of the basic oxygen process it
is often accompanied by a significant increase in dissolved
nitrogen content. The amount of this increase will vary.
Typical nitrogen pickup during conventional reblowing
is in the range of 2 to 10 ppm, with increases of up to
15 or 20 ppm not uncommon. Further, if several reblows
in succession are required, the final nitrogen level may
be as much as 80 to 100 ppm higher than N* and 40 to 60
ppm higher than the maximum acceptable level for some
grades of low-carbon, low-nitrogen steel.
It is believed that the reason for such high nitrogen
pickup is that whîle refining is temporarily stopped,
atmospheric nitrogen diffuses in~o the vapor or head-
space of the vessel and is absorbed by the melt during
the subsequent reblow. In accordance with this invention,
nitrogen is removed from the vessel by purging the
vessel with a nitrog~n-free fluid, just prior to starting
the reblow and by maîntaining the off-gas flow rate no
lower than F* during the reblow. While any amount of
purging wîll be helpful it has been found that purging
with a volume of gas (measured at 70F and atmospheric
pressure) approximately equal to half the total volume
of the headspace of the vessel is sufficient to minimize
the nitrogen pickup by the steel during the reblow.
Purging with less inert gas is likely to be insufficient,
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` D-10,883
37971
while purging with more is technically accPptable but
uneconomical. It should be noted that if multiple
reblows are required, the vessel must be purged prior
to each reblow.
Argon is the preferred nitrogen-free fluid for use
in the present invention. This gas has the advantages
of being inert chemically, of being the least expensive
and most abundant of the chemically inert gases, of
being the least disruptiv~ to the thermal balance in
the vessel, and also of favorably affecting the reaction
of oxygen with carbon by diluting the effluent carbon
monoxide. Other nitrogen-free gases can also be used,
as well as liquids which vaporize readily at steel
refining temperatures. Examples of other nitrogen-free
fluids include, but are not limited to: helium, neon,
krypton, xenon, carbon dioxide, carbon monoxide, steam,
water, hydrogen, methane, liquid hydrocarbons, gaseous
hydrocarbons,or mixtures thereof, including mixtures with
argon.
When using a flammable gas such as methane or
hydrogen, special precautions should be taken to avoid
forming an explosive mixture prior to injection into
the refining vessel. The flammable gas will, of course,
react with oxygen in the vessel. This reaction must
be taken into account when calculating the amount of
- off-gas that will be produced for each quantity of
D-10,883
~ ~375~71
flammable gas added.
In order to best attain the further benefits of
minimizing the amount of metal oxidized, and of reducing
the amount of oxygen dissolved in the melt, the preferred
means for injecting the nitrogen-free fluid into the
vessel is to mix it with the oxygen, if that can be
accomplished without forming an explosive mixture. By
using argon the possibility of creating an explosiv~
mixture is entirely eliminated. By injecting ths
nitrogen-free fluid admixed with oxygen, the invention
may be practiced on existing BOP systems with very
little investmen~ since there is no need to add new
injection equipment~ It is possible simply to meter
the nitrogen-free fluid into the oxygen line at
some point upstream of the oxygen lance. However,
it is also possible to practice the invention by
~njecting the nitrogen-free 1uid by a separate
injecting lance, tuyere, or other injecting means
located any place in the vessel, be it in the headspace,
below the surface of the melt, or as a separa~e conduit
within the oxygen lance.
D-10,883
~ ~ 7 ~7 ~
The following examples will serve to illustrate
the practice of the present invention.
, EXAMPLES
Several steel heats were refined by top blowing
in a BOP refining system having the following characteristics:
Vessel volume 5000 ft3
Vessel mouth area 95 ft2
Total charge (pig iron and 235 tons
scrap metal)
Average æmount of pig iron in charge 162 tons
Average pig iron composition 4.5% carbon
1.0% silicon
- 0.8% manganese
Nitrogen-free fluid Argon gas
Oxygen blowing rate Without 20,000 ft3/min
argon: (at 70F and 1 atm)
with 16,500 ft3/min
- argon: (70F and 1 atm)
Off-gas temperature 2900F
The size of the lance limited the total flow
rate of injected gas such that the oxygen blowing rate
had to be reduced while argon was being injected. The
in~ention is preferably practiced by maintaining a
constant oxygen blowing rate throughout the entire heat.
D-10,883
~ 7 ~ ~ i
The graphs relating nitrogen content and off-gas
flow rate for this vessel with carbon content of the
melt are shown in Figures 1 and 2. From the graphs it
can be seen that the minimum nitrogen level, N*, occurs
at a carbon content of approximately 0.08% and an off-
gas rate of 15,000 ft3/min (measured at 2900F and
1 atmosphere or pressure). Thus, in order to properly
practice this invention, the latest poir.t in time for
introduction of nitrogen-free fluid into the vessel, is
at a nitrogen content of about 19 to 25 parts per million
or a carbon content of 0.08%. The argon must be injected
at a rate sufficient to maintain the off-gas rate at
15,000 ft3/min measured at 2900F and 1 atmosphere, or
about 2300 ft3/min measured at 70F and 1 atmosphere.
Argon was introduced into the BOP vessel via the
oxygen lance by metering argon into the oxygen supply
line upstream of the lance. Since a precise means to
continuously measure the nitrogen or carbon content of
the melt during the refining process was not available,
the argon flow was begun when the carbon content was
estimated to be between 0.10% and O~15~o~ To maintain
an off-gas rate of 15 ~000 ft3/min at 2900F, 3000 ft3/min
of argon measured at 70F, or 19,000 ft3/min at 2900F,
was injected. The extra gas was added to provide a
safety factor in case all the argon was not heated to
2900F. Some runs were performed with argon added at
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- D-10,883
~1~'7~71
a constant rate as low as 2000 ft3/min (at 70~F and 1
atm). These runs also gave satisfactory results.
Table 1 shows the results obtained upon the
first stoppage of oxygen or first turn down, for heats
in which reblowing was not required prior to the time
that argon was added to maintain the off-gas flow rate.
TABLE 1 -_ NITROGEN CONTENT AT_FIRST TURNDOWN
Heat No: 1 2 3
-
Argon rate (ft3/min
at 70F and 1 atm) 0 2000 3Q00
Duration of total
oxygen blow (minutes) 17 17 16
Duration of argon
injection (minutes) 0 4.25 2.00
Temperature (F)2880 2935 2890
Carbon content at
first turndown ~/O) O.03 0.03 0.03
Nitrogen content at
first turndown ~parts
per million) 33 20 2
The results in Table 1 show the lower nitrogen
content obtained while practicing the invention in Heats
No. 2 and 3 as compared with Heat No. 1, during which the
invention was not practiced.
Table 2 illustrates the efect of purging the
vessel prior to a reblow. In these heats argon was not
introduced into the vessel prior to the first turn down.
It was used to purge the vessel prior to the reblow
and also added to the oxygen during each reblow. It
-17-
D-10,883
~ ~ ~ 7 ~ 7 1
is evident that purging the head space followed by
addition of argon to the oxygen during the reblow
essentially eliminates pickup of nitrogen even when the
carbon content is as low as 0.03~/0. Consider, for example,
Heat No. 1 where the purpose of the reblow was to raise
the melt temperature. The carbon content was 0.03% both
before and after the reblow- i.e. there was little or
no carbon removal and hence there would, in the absence
of argon, be little or no off-gas. Because the vessel
was first purged with argon and then reblown with oxygen
plus argon the total nitrogen pickup during the reblow
was minus 1 ppm, i.e. the nitrogen level actually
decreased. At this low carbon lev 1 one would anticipate
a nitrogen pickup of at least 5 ppm if argon purging and
argon addition during the reblow had not been practiced.
; Heat No. 4 is an example of a heat where multiple
reblows were required. Argon purging was used prior to
each reblow and argon was added to the oxygen during
each reblow. Again it is evident from the results
; 20 shown in Table ~ that the addition of argon in accordance
with this invention resulted in a cumulative nitrogen
pickup of minus 3 ppm (i.e. a nitrogen decrease) after
four consecutive reblows. Normally, at these low carbon
levels in the absence of argon addition, one would
anticipate a minimum cumulative nitrogen pickup of about
20 ppm after 4 reblows, and a total pickup of 40 to 60
ppm would not be unusual.
-18-
~137~71
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D-10,883
~ ~ ~ 7 ~ 7 i
Table 3 illustrates the results of practicing the
invention when it is necessary to reblow a heat after
argon addition to maintain the minimum off-gas flow rate
prior to first turn down. In Heat No. 6, argon flow
was initiated at a rate of 2000 SCFM 390 seconds prior
to the first turn down. At turn down the temperature
was 2950F, carbon 0.13/~ and nitrogen 16 ppm. The
vessel was then purged with 2500 SCF of argon and
reblown for 60 seconds with 16,500 SCFM oxygen and 3000
SCFM argon. After 60 seconds the temperature was 2860F,
carbon was 0.07% and nitrogen was 19 ppm. The vessel
was again purged with 2500 SCF argon and again reblown
for 60 seconds with 3000 SCFM argon and 16,500 SCFM
oxygen, and at turn down the temperature was 2910F,
carbon was 0.04~/O and nitrogen, 18 ppm. Total nitrogen
pickup during the two reblows was 2 ppm. The heat was
then tapped.
Heat No. 7 is similar to Heat No. 6 except that
only one reblow was required, and the nitrogen pickup
was minus 2 ppm, i.e. the nitrogen level decreased.
-20-
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